Daily or circadian rhythms controlled by endogenous clocks and synchronized to the solar cycle are characteristic of all organisms. The primary mammalian circadian clock in the suprachiasmatic nuclei (SCN), survives in vitro where it continues to produce 24 rhythms. SCN clock phase is modulated by afferents from the retina, intergeniculate leaflet of the lateral geniculate nucleus, and raphe nuclei. The primary neurotransmitters for these inputs are an excitatory amino acid (possibly glutamate), neuropeptide Y (NPY) and gamma-aminobutyric acid (GABA), and serotonin (5-HT), respectively. These afferents synapse onto vasoactive intestinal polypeptide (VIP)-containing SCN cells, and possibly converge onto the exact same cells. Thus, modulation of clock phase may involve an interaction among these afferent systems, possibly at the level of the VIP cells. I am currently investigating interactions between these afferent systems in terms of their ability to phase shift the clock and to modulate SCN cell firing. As part of this ongoing effort, I propose to investigate how these afferent neurotransmitters affect VIP release in the SCN in vitro. This will be the first systematic investigation of how these afferent systems affect VIP release in the SCN. As such it could provide critical information concerning how afferent signals are integrated within the SCN, and ultimately how they modulate SCN clock phase. The increased knowledge of how the SCN circadian pacemaker can be manipulated by external stimuli should produce rapid advances in our ability to alleviate problems that have been linked to circadian rhythm disorders, including sleeplessness, narcolepsy, and manic depression, as well as the medical and performance problems associated with jet lag and shift work schedules.